This invention relates to radio antennas and more particularly to a highly unique type of 5/8 wavelength two-way radio antenna designed as the solution to a chronic radio interference problem encountered by amateur radio operators using two-way mobile radio communications in the 146-148 Mhz portion of the 2 meter amateur radio band. For brevity, this band will be referred to as the 146 Mhz band.
The source of the chronic interference referred to above, is due to commercial, government and broadcast stations operating at frequencies above and below the 146 Mhz band. A very frequent and major source of interference is from the 150 to 170 Mhz frequency band where there are hundreds of radio channels assigned to two-way radio communication systems such as in use by business radio, telephone company, police and fire departments, hospital and ambulance, taxicab, municipal, county, state, federal, marine ship to shore et al. Other sources of interference to the 146 Mhz band receivers are aircraft two-way radio operating in the 110 to 140 Mhz band, F.M. broadcast stations in the 88-108 Mhz band and the television broadcast channels above 170 Mhz. These operations normally should not cause interference in 146 Mhz band radio receivers, providing that the receiving equipment is selective enough in design and construction to provide ample attenuation to all unwanted radio frequencies. However, in too many cases, mainly because of manufacturing cost considerations and space limitation, the receiving equipment is not adequate in that respect.
Most of the interference is caused by the close proximity and/or high intensity of the interfering radio signal getting into the R.F. input circuit, commonly known as the "front end" of the radio receiver, thereby mixing with other external and internal signals, causing what is known as intermodulation product interference, usually referred to as "intermod." These unwanted signals getting into the front end of the radio receiver can also cause overloading or blocking and will tend to greatly reduce the sensitivity of the receiver, preventing the reception of all but the strongest desired signals. This condition is referred to as "desensitization."
A common method that has been very successful in minimizing or completely eliminating the types of interference discussed, has been the use of cavity resonators as highly selective narrow bandpass filters placed at the receiver RF input terminals, or incorporated as an integral part of the radio receiver. Basically, a cavity resonator is an enclosure or partial enclosure of any size and shape, having conducting walls or surfaces that can support oscillating electromagnetic fields within it and possesses certain resonant frequencies when excited by electrical oscillations. The basic paper on cavity resonators was presented by W.W. Hansen, "A Type of Electrical Resonator," Journal of Applied Physics, Vol. 9, page 654, October 1938.
The most common type of cavity resonator used in the type of interference elimination discussed above, is the re-entrant cavity resonator, consisting of a 1/4 wavelength inner conductor placed concentrically in a cylinder type enclosure. One end of the inner conductor is connected to one end plate and the other end is left open, but in close proximity to the other end plate. Resonance at a particular frequency is achieved by mechanical adjustment of the length of the inner conductor within the cavity housing. Often, to conserve space, the entire assembly is shortened to less than a 1/4 wavelength and an adjustable capacitor is added to the normally open end to tune the cavity to resonance. The re-entrant type of cavity can be further reduced in size by coiling the inner conductor into a small helix and placed within a small cavity housing. We then have a hybrid type of re-entrant cavity referred to as a helical cavity resonator. Two such resonators are often coupled together for greater selectively. One of the basic papers on this type of resonator was presented by W. Macalpine and R. Schildknecht, "Coaxial Resonators with Helical Inner Conductor," Proceedings of the Institute of Radio Engineers, December 1959, page 2099.
The input and output circuits of the re-entrant cavity are usually designed to match the impedance of commonly used 50 ohm coaxial cable. This is done by providing small pick-up loops placed in the electromagnetic field or by direct connection to the inner conductor. Higher impedance inputs and/or output can be obtained by using larger pick-up loops or by tapping directly to the inner conductor at higher impedance points. A combination of both are often used. Re-entrant type cavity resonators can be designed in various sizes, shapes and configurations. The interior surface of the cavity housing and the exterior surface of the inner conductor should afford high conductivity of electric currents. The inner conductor can be solid, hollow, round, square or flat and either be of 1/4 wave resonant in length or, if shorter, be tuneable to the desired resonant frequency. The various types of re-entrant cavities have acquired many descriptive names. To name a few; resonant cavity, resonant bandpass filter, cavity resonator, coaxial bandpass filter, coaxial filter, coaxial tank, trough-line filter, combine filter, interdigital bandpass filter, stripline filter, helical cavity resonator, helical resonator, et al. They all have one thing in common; they are all in the resonant cavity family and can be used to provide highly selective bandpass filtering.
Conventional inductance and capacitor type of resonant circuits made up of lumped circuit constants are not practical for use in bandpass filters at very high frequencies (VHF) to obtain the degree of selectivity required to eliminate intermod and desensitization type of interference. In many cases they carry descriptive names which could be confused with re-entrant resonant cavities, such as bandpass filter, resonator, tank circuits et al. However, they all have one thing in common; they are not in the resonant cavity family.
Coaxial cables, when cut to an electrical 1/4 wave or 1/2 wave in length, or multiples, are often used as resonant circuits. They are neither lumped constant resonant circuits nor are they in the resonant cavity family but bridge the gap between them.
When both input and output circuits are provided for in a re-entrant type of cavity, the circuit is equivalent to a parallel resonant circuit offering high impedance bandpass characteristics to the desired resonant frequency. Conversely, when only one input is used, the circuit becomes the equivalent of a series resonant circuit and offers very low impedance to the resonant frequency and is usually used as a wave trap or suck-out filter.
A combination of the two types of useages is usually where the transmitter and receiver are both used simultaneously and fairly close in frequency such as in repeater type of operation or for duplex telephone operation. This combination is usually referred to as a duplexer.
Cavity resonators are often used as a tank circuit in a radio transmitter, or externally to it, to prevent radiation of spurious emmissions and to prevent re-radiations of unwanted signals that might gain entry otherwise.
In 146 Mhz mobile two-way radio communication, as used by radio amateurs, one of the most widely used antennas is the 5/8 wave whip antenna. The reason for its popularity is because it concentrates most of the radiated energy into the low angles so desirable for transmission and reception at this frequency. The higher angle radiation pattern of the conventional 1/4 wave ground plane whip antenna wastes a great deal of energy into space. Unfortuantely, a 5/8 wave whip antenna will not resonate without the addition of inductive reactance. Therefore this type of antenna is manufactured using a small series coil at its base, usually sealed in a plastic housing and designed for a particular band of frequencies. In many cases the coil is made tuneable to the desired operating frequency band, by introducing capacity and varying one in respect to the other or both as is the case of the variable sleeve type of base coil where the sleeve is moved over the coil in micrometer fashion to resonate the antenna. All these antennas are of the broad band variety and do not in any way act to reduce intermod or desensitization type of interference.